It is well known that some sheet materials are extremely difficult to fold in a precise and uniform manner along a predetermined fold line because the nature of the material deforms irregularly as the material is being folded. This problem is particularly severe in the case of cardboard sheets and corrugated cardboard sheets which tend to buckle at the fold line irregularly and cause the folded portion to become misaligned relative to the body portion of the sheet. In the case of corrugated blanks which have panels to be folded in order to form a container, the accuracy of the fold and the precise alignment of the folded panels is absolutely critical to the formation of a successful product. That is, any deformation or misalignment of the panels changes the shape of the container and may make it unacceptable. When folded in precise alignment, a container blank is said to be "square" or "squared".
This problem and the need to square each folded carton blank is discussed, for example, in U.S. Pat. No. 2,986,078 including illustrations of squared and skewed carton blanks, and this patent is hereby incorporated by reference.
Many attempts have been made to solve this problem, but none have totally eliminated the problem, and current speeds of folding up to 1,000 board feet per minute of cardboard blanks have made it even more difficult. All known prior attempts have utilized rods or belts to perform the folding action. These only make point or line contact with the flap of the panel, and the force is applied at an acute angle with respect to the fold line which is also the direction of travel of the blank. In addition, the initial contact of the rod or belt is with the leading edge of the flap. Both of these factors tend to bend the flap and/or twist it as the panel is being folded. Thus, there remains a critical need for a high-speed folder which can produce highly squared container blanks with a substantially lower product rejection rate.